The present invention relates generally to communication devices, and more particularly to radio frequency identification (RFID) devices providing increased tag activation distances.
In general, an RFID system consists of one or more tags, a tag reader, and a host computer system. Tags are devices that can come in many sizes and form factors, but are usually small and lightweight. Tags are commonly used as portable data devices that wirelessly communicate with RFID readers at distances ranging from a few millimeters to several meters. The information stored in a tag can be used, for example, to identify an individual or object carrying the tag.
RFID technology is used in a variety of applications because of its convenience and flexibility. An example application for RFID technology is a building security system. As part of a building security system, RFID systems are used to grant access only to individuals carrying authorized tags (or cards). When an individual places their card in the vicinity of the reader, the reader interrogates the card and obtains identification information stored in the card. After further processing, the reader communicates the individual""s identification (xe2x80x9cIDxe2x80x9d) code to a host computer in the security system. If the ID code received by the host computer system is authorized, the door is unlocked to permit access to the building.
RFID systems are also used to detect specific items and link those items with other information and events. RFID systems can be used, for example, to track products being built in a factory, to trigger manufacturing steps to occur, to assist in inventory control, etc. Read-only tags are ones in which the data is programmed once, and the tag only sends the stored information to the reader. Read-write tags have the ability to be reprogrammed to suit the needs of the application. Therefore, read-write tags can be used as portable databases, eliminating the need for central databases.
Most RFID tags contain functional electronics in the form of an integrated circuit, or IC, to store and process data, and to perform communication functions. RFID tags also contain an antenna, which is used as the radio frequency interface with the reader. The IC requires power to operate, which can be supplied by a battery. Most applications, however, require tags to be small and inexpensive, so batteryless, or xe2x80x9cpassivexe2x80x9d, tags are in very wide use. Passive tags receive energy from the radio frequency (xe2x80x9cRFxe2x80x9d) field generated by a reader, and the IC converts the RF to direct current (xe2x80x9cDCxe2x80x9d) operating power for itself. Once operating, the IC communicates with the reader, which has an antenna system for transmission and reception of signals. Power and data are transferred between tag and reader through one or more antennae in each device. The reader antenna used to couple power and/or information from the reader to the tag is called the exciter antenna. The reader antenna used to receive information from the tag is called the receive antenna.
A key performance parameter of all RFID systems is read range. Read range is the distance between the card and reader at which the reader captures the data transmitted by the card. Read range is a function of two factors. Tag activation distance is the distance between the tag and the reader at which the tag receives sufficient energy to power the IC. The tag can then begin to send data to the reader. Receiver sensitivity controls the distance at which the reader can receive the transmitted data. If receiver sensitivity is low or is compromised by interfering signals the tag must be moved closer to the reader until the data signal strength exceeds the reader receiver sensitivity. Internally or externally generated noise may effectively reduce the sensitivity of the receiver. Thus the read range of an RFID system can never be greater than the tag activation distance.
Some tag-reader systems communicate via magnetic fields, while other types of systems communicate via electric fields. Electric field tags offer advantages in cost, size, weight and flexibility compared with magnetic field tags. Many applications demand small, compact and inexpensive readers, as well. Shrinking the size of electric field RFID readers, however, presents unique design challenges. Without addressing these challenges, reader performance is significantly impaired.
Two types of electric field RFID reader and tag systems exist, and are referred to as monopole and dipole systems. A monopole electric field RFID reader, or RFID device, has a single exciter antenna, or exciter electrode, driven by a voltage source that is referenced to an impedance that is common with the environment (xe2x80x9ccommon impedancexe2x80x9d) in which an electric field tag is being used. The tag has two antennae, or electrodes, the first of which is preferentially coupled to the impedance that is common with the RFID device. For example, the preferential coupling impedance may be formed by a person holding the tag, while the common impedance may be formed by earth ground. The total return impedance, defined as the sum of the preferential coupling impedance and the common impedance, may be resistive, capacitive, inductive, or any combination thereof. When the tag is close enough to the RFID device, displacement current will flow from the exciter electrode to the second tag electrode through the capacitance that exists between them. Current will then flow through the tag IC, and back to the reference terminal of the exciter voltage source through the total return impedance. If sufficient current flows, the tag IC will become activated. In general, the total return impedance is much lower than the reactance of the very small capacitance that exists between the exciter electrode and the second tag electrode. Therefore, the displacement current that activates the tag is limited primarily by the small capacitance between the reader and the tag.
Dipole electric field RFID devices contain two exciter electrodes whose voltages are opposite in polarity but balanced about a common impedance path such as earth ground or the chassis of equipment. Dipole systems do not require preferential coupling of one tag electrode to a common impedance of the system, although preferential coupling may be utilized in some cases. Because the pair of tag electrodes is coupled to the pair of RFID device exciter electrodes through a series combination of two small capacitance values, the effective tag-reader coupling impedance is much larger in dipole systems than in monopole systems. Therefore, the tag activation distance of a dipole electric field RFID system is substantially smaller than that of a similarly sized monopole electric field RFID system. Monopole electric field RFID systems are used in more applications than are dipole systems because of the improved coupling efficiency and large tag activation distance that is achievable.
For simplicity, the following discussion will describe electric field RFID devices in terms of monopole systems, even though the concepts also apply to dipole systems. For clarity, only the excitation function of electric field RFID systems will be described. Other functional elements that are required for full RFID device functionality, such as the receive electrode(s), receiver, demodulator, decoder, processor, I/O circuitry, etc., are understood by those skilled in the art, and are not relevant to this discussion.
FIG. 1 is a simplified side pictorial/schematic view of a system containing the electric field RFID device 14 and the tag 5, illustrating the excitation portion of the system. In FIG. 1, the exciter antenna assembly 1 is comprised of the exciter electrode 2 (e.g., antenna plate, etc.) which is illustrated as an electrically conductive layer, or sheet, disposed upon the dielectric substrate 3, which can be fabricated using well-known technologies such as, but not limited to, printed circuit board technology. The exciter voltage source 4 generates a high alternating current (xe2x80x9cACxe2x80x9d) voltage that is connected to exciter electrode 2. Exciter electrode 2, driven by exciter voltage source 4, causes an AC electric field to be radiated outward toward tag 5. When tag 5 is close enough to exciter electrode 2, the displacement current 6 flows through the coupling capacitance formed between exciter electrode 2 and the tag electrode 52. Displacement current 6 becomes a conduction current and flows through the IC 51 of tag 5, the tag electrode 53, the preferential coupling impedance 30, the common impedance 7 (such as, but not limited to, earth ground), and the electric field RFID device reference connection 9, ultimately returning to exciter voltage source 4 at the exciter voltage source return node 91. If sufficient current is coupled from exciter electrode 2 to tag 5, tag 5 will begin to function. Therefore, displacement current 6 provides operating power for tag 5. Relatively high voltage levels must be present on exciter electrode 2 in order to produce an adequate magnitude of displacement current 6 when tag 5 is at long distances from exciter electrode 2. Electric field RFID device reference connection 9 and common impedance 7 are shown connected to the system ground 8. It should be noted that FIG. 1 is not drawn to physical scale, that is, tag 5 is typically positioned at a much greater distance from the exciter electrode 2 than is suggested in FIG. 1. Tag activation distances can range from a few inches to several feet depending upon the size of the tag electrodes, the size of the electric field RFID device exciter electrode and the exciter voltage. Although a flat planar exciter electrode is illustrated in FIG. A, surfaces having complex shapes can also be used. A receive electrode (not shown) is often located near the exciter electrode 2 for the purpose of receiving signals from tags.
The charge distribution for one phase of the exciter voltage source is illustrated in FIG. 1. A high concentration of positive charge appears on the outer surface (closest to tag 5) of exciter electrode 2, while a high concentration of negative charge appears on tag electrode 52, which is nearest to exciter electrode 2. When the phase of the exciter voltage source reverses, the polarities of the illustrated charge also reverse. FIG. 1 represents an idealized case, which is often not realizable in many practical applications.
In many practical systems, other electrically conductive surfaces and elements are usually in the vicinity of exciter electrode and the tag. This can occur, for example, as a result of conductive materials being present in or behind the surface to which the exciter electrode assembly is mounted, as shown in FIG. 2. In capacitively coupled systems, this causes a re-distribution of the electric charges and associated electric fields. The presence of the electrically conductive surface 11 behind exciter electrode 2 causes the displacement current 19 to flow from exciter electrode 2 to electrically conductive surface 11, and pulls the electric field lines from exciter electrode 2 towards it, reducing the electric field strength that radiates outward towards tag 5. This reduces the amount of charge that is available on the outer surface of exciter electrode 2 for coupling to tag 5, thereby causing a reduction of the coupled power. The reduction of charge on the outer surface of exciter electrode 2 is indicated by fewer + charge symbols on the outer surface of exciter electrode 2, when compared to the inner surface of exciter electrode 2, and as compared to the outer surface of exciter electrode 2 in the idealized case of FIG. 1. Tag 5 requires minimum values of voltage and current to become activated, and the maximum distance at which tag 5 becomes activated is referred to as the tag activation distance. However, if power coupled to tag 5 is reduced by the presence of nearby conducting surfaces, the tag will no longer receive enough power to be activated. As a result, tag 5 must be located closer to exciter electrode 2 in order to receive sufficient power for activation.
The net effect is that the tag activation distance is reduced by the presence of nearby conductive surfaces, as illustrated in FIG. 3. Exciter electrode 2 is disposed within the enclosure 23, which is attached to the mounting surface 200. Mounting surface 200 may be a wall, table, shelf, etc. For mounting surfaces composed of dielectric materials, the first maximum tag activation boundary 100 is obtained. Tags may be activated at and within the space defined by this boundary. Although first maximum tag activation boundary 100 may also extend below mounting surface 200, it is not illustrated for clarity because in many cases, a tag cannot be physically positioned in this region. Next, consider the case in which mounting surface 200 is composed of an electrically conductive material. Electric charge is redistributed and becomes concentrated between exciter electrode 2 and mounting surface 200, thereby reducing the electric charge available for coupling to tags. The result is depicted by the second maximum tag activation boundary 101. The tag activation distance in front of and to the sides of exciter electrode 2 is reduced substantially. The electric field lines radiated by exciter electrode 2 terminate on mounting surface 200 because is conductive, and second maximum tag activation boundary 101 does not extend below mounting surface 200.
Other types of nearby conducting surfaces are also present in many practical implementations of the reader. It is often desired that readers be as small as possible, and constructing such compact readers means that some or all of the functional electronics must be contained within the same enclosure as the exciter electrode.
FIG. 4 is an example of the electric field RFID system 13 having the compact electric field RFID device 14. Electric field RFID device 14 is composed of two basic elements, an exciter electrode (e.g., antenna, plate etc.)2 and the electronic circuitry 15. Electric field RFID device 14 may be any part of an RFID reader system containing tag excitation circuitry, such as, a tag reader, a tag writer, a tag reader/writer, a tag excitation device (in which the circuitry that performs the tag reading function is located in a separate unit), or any combination thereof. Exciter electrode 2 is a sheet of electrically conductive material. Electronic circuitry 15 contains all of the functional circuitry required to drive exciter electrode 2, communicate information between tag 5 and electric field RFID device 14, and exchange information with the host computer system 17 via the input/output (xe2x80x9cI/Oxe2x80x9d) cable 16. Power can be provided to electric field RFID device 14 by host computer system 17 via I/O cable 16, or electric field RFID device 14 can contain a portable power source (not shown). Electronic circuitry 15 is assembled on the substrate 18 comprised of a dielectric material, such as, epoxy glass printed circuit board (PCB). Alternatively, substrate 18 may be made of a wide variety of materials, such as, polymer sheets or films, paper or cardboard, ceramic, etc. Components used in electronic circuitry 15 are interconnected by the conductors 40 on substrate 18. The conductors are formed of metals, metal foil, metal film, electrically conductive inks or paints, etc., and may be constructed using any suitable means, such as deposition and etching.
The presence of the additional wiring and electronic components forms conductive surfaces near the exciter electrode as described previously, and tag activation distance will be reduced from the idealized case illustrated in FIG. 1 as a result. Furthermore, the close proximity of the reader circuitry will cause displacement currents to flow from the exciter electrode into the nearby circuitry. The injected displacement current 19 that flows through sensitive circuits in electronic circuitry 15 can significantly reduce sensitivity to the tag signal. This reduction of read path sensitivity causes the read range to drop to levels well below that of the tag activation distancexe2x80x9470% decreases have been observed in some cases. In addition, the displacement current can cause abnormal functional behavior and can increase unintentional conducted and/or radiated emissions from the reader, posing radio frequency compliance problems.
To contend with the adverse effects of internally injected displacement current, a displacement current control surface may be used quite successfully as described in U.S. Pat. No. 6,229,442. FIG. 5 illustrates the introduction of the displacement current control surface 20 into the system of FIG. 4. However, the introduction of a displacement current control surface can increase the overall area of conductive surface(s) near the exciter electrode even further. The increase in nearby conductive surface area can cause a further reduction in tag activation distance, as indicated by very few + charge symbols on the outside of exciter electrode 2. The more compact the reader must be, the more a displacement current control surface is needed to preserve receive sensitivity. This comes, however at the loss of tag activation distance. With the inclusion of a displacement current control surface, it is quite possible for the receive sensitivity to yield a maximum read distance capability that is greater than the tag activation distance. However, the effective system read range can never be greater than the tag activation distance. Therefore, a loss in tag activation distance will also reduce effective system read range.
The decrease in tag activation distance that occurs as a result of nearby conductive surfaces can be overcome by increasing the exciter voltage. However, this usually comes at the expense of increased reader power consumption and increased internal operating temperatures, thereby limiting the operating temperature range of the product or compromising its reliability. Increasing the exciter voltage can also increase internal noise within the reader, potentially reducing receive path sensitivity. Creating circuits to increase the exciter voltage may also increase conducted and/or radiated emissions, posing regulatory compliance difficulties.
Another compensation method is to increase the area of the exciter electrode. This approach has the disadvantage of increasing RFID device the package size. Alternatively, reader packaging can be made larger such that the closest nearby conductive surfaces are maintained at greater distances from the exciter electrode. The reader could be packaged in two physical enclosures, allowing the idealized case to be more closely approximated. Still, the exciter electrode enclosure would have to be large enough to render the effects of conductive materials in the mounting surface to be acceptably small. Larger single-piece readers or even two-piece readers can easily be more costly and weigh more; and incur additional shipping cost. If utilized, these compensation methods can cause the resulting RFID devices to be unsuited for many applications because of excessive product size, cost, installation difficulty, etc.
Thus, there exists a need to provide an apparatus and method for increasing the effective electric field strength of RFID devices to activate RFID tags at greater distances. A low-cost solution that enables compact electric field RFID devices to function well and predictably in a wide variety of applications is desired.